Compression ignition engines provide advantages in fuel economy, but produce both NOx and particulates during normal operation. New and existing regulations continually challenge manufacturers to achieve good fuel economy and reduce the particulates and NOx emissions. Lean-burn engines achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust of these engines yields significantly high concentrations of NOx as well. Accordingly, the use of NOx reducing exhaust treatment schemes is being employed in a growing number of systems.
One such system is the direct addition of ammonia gas to the exhaust stream. It is an advantage to deliver ammonia directly in the form of a gas, both for simplicity of the flow control system and for efficient mixing of reducing agent, ammonia, with the exhaust gas. The direct use of ammonia also eliminates potential difficulties related to blocking of the dosing system, which are cause by precipitation or impurities, e.g., in a liquid-based urea solution. In addition, an aqueous urea solution cannot be dosed at a low engine load since the temperature of the exhaust line would be too low for complete conversion of urea to ammonia (and CO2).
Transporting ammonia as a pressurized liquid, however, can be hazardous if the container bursts caused by an accident or if a valve or tube breaks. In the case of using a solid storage medium, the safety issues are much less critical since a small amount of heat is required to release the ammonia and the equilibrium pressure at room temperature can be—if a proper solid material is chosen—well below 1 bar. An ammonia adsorbing/desorbing material can be provided in the form of disks or balls loaded into the cartridge or canister. The canisters are then loaded into a mantle or other storage device and secured to the vehicle for use. Appropriate heat is applied to the canisters, which then causes the ammonia-containing storage material to release its ammonia gas into the exhaust system of a vehicle, for example.
However, during engine start-up, there is generally insufficient heat to activate the ammonia storage material in the main canisters to the point of releasing its ammonia gas. Therefore, the present system and method incorporates a start-up cartridge, which can be quickly heated at start-up providing an initial release of ammonia gas into the exhaust stream, until the main cartridges are activated. In addition, because the start-up cartridge is positioned separate from the main cartridges, the start-up cartridge cools faster than the main cartridges, and can be recharged faster with ammonia gas than the main cartridges. Thus, the present system and method provides for quicker activation of the NO reduction cycle even at start-up. Additionally, the insulated mantle housing is an efficient thermal barrier maintaining required temperatures around the main ammonia-containing cartridges for continued ammonia gas release into the exhaust system.